Skipping right to the fix... the issue is in this line of code here:

There's a missing check to ensure that the slot is not null before calling getHasStack() on it.

Crash report demonstrating the crash: https://pastebin.com/CLx5FpMT

Reproduced with:
forge-1.16.5-36.1.0
engineersdecor-1.16.4-1.1.9

Hi man, thanks for digging into the code directly! Gonna see to prepare and push the fix at the weekend. Cheers!

Oky, included fix in the current maintenance release. File is on Curse. Ty again for reaching out, cheers,-